Raw natural gas and other hydrocarbon streams often contain naturally occurring contaminants, such as, for example, water vapor, CO2, H2S, and mercaptans, and other sulfur compounds. In the case of raw natural gas, other contaminants, such as methanol or glycol, are sometimes purposely added at the natural gas field production facility to prevent the formation of hydrates or ice in the natural gas production stream while it is in transit to the treating facility. Whether such contaminants occur naturally in the gas or are purposely added, they must be substantially removed prior to use of the natural gas in certain industrial or residential applications.
One method of removing sulfur contaminants from liquid hydrocarbon streams in the refining industry includes passing the streams over beds of metal impregnated adsorbents. In U.S. Pat. No. 5,157,201 metal oxides are used to adsorb sulfur from a propylene/propane stream without using hydrogen. An olefin stream is heated to about 50° C. to 175° C. at a pressure of about 175 psig to 1100 psig and passed over a commercial CoMo (cobalt-molybdenum) oxide adsorbent to remove sulfur contaminants. The adsorbents are said to be regenerable using a mixture of air and steam at 400° C. In U.S. Pat. No. 6,579,444 hydrocarbon streams boiling in the naphtha range have sulfur compounds removed by a first step of hydrodesulfurization in the presence of any of a number of metal compounds (preferably cobalt and molybdenum sulfided compounds) and a second step of contacting the streams with adsorbents comprised of cobalt and at least one of molybdenum or tungsten (principally the metal oxides) on a refractory support at temperatures of, preferably 10° C. to 100° C., and pressures of from atmospheric to about 500 psig. Regeneration is with any suitable regenerant, including nitrogen, a mixture of hydrogen and hydrogen sulfide, as well as organic solvents, both aromatic and non-aromatic. U.S. Pat. No. 7,074,324 teaches the removal of sulfur compounds from hydrocarbon streams, especially gasoline, by contacting with adsorbent materials that are regenerated with hydrogen or a hydrogen/H2S mixture. The adsorbent material is selected from any hydrotreatment compound containing a least one Group VIII metal, preferably selected from Fe, Co and Ni, alone or in combination with at least one Group VI, IA, IIA, IB, or mixtures, preferably supported, e.g., on alumina. Preferably the compound is CoMo, and supports generally include zeolitic compounds. Regeneration uses hydrodesulfurization conditions, e.g., pressures from about 0 to about 2,000 psig and temperatures from about 100° C. to about 600° C.
Principal methods of removing contaminants from gaseous hydrocarbon streams, including raw natural gas streams, involve the use of solid adsorbents which include, for example, alumina, silica gel, activated carbon and molecular sieves such as zeolites. These materials are typically used in packed beds. Typically, a contaminated hydrocarbon stream is passed through the bed and the adsorbent materials in the beds adsorb the contaminants preferentially, thereby reducing their concentration in the hydrocarbon stream effluent emerging from the bed.
The adsorbents eventually become saturated with adsorbed contaminants, at which point the adsorbent will no longer effectively remove the contaminants from the hydrocarbon stream. When saturation occurs, the adsorbent materials must be either replaced or regenerated. One way of regenerating an adsorbent is to pass a heated regeneration fluid stream, either in a gaseous or a liquid state, through the adsorbent bed, often in a countercurrent manner. In this way, the adsorbed contaminants are desorbed from the adsorbent and moved into the regeneration fluid stream in which they are carried out of the bed. The regeneration fluid stream can then be purified and recycled, or it can be used as fuel gas.
During regeneration, temperatures in the beds can often reach approximately 600° F. (315.6° C.). At these temperatures, particularly in the case of zeolitic molecular sieves, some adsorbed species may “crack” to form highly carbonaceous compounds, or “coke.” For example, while water and some other compounds are simply desorbed from the molecular sieves during regeneration, alcohols, glycols, heavy hydrocarbons such as benzene, toluene, and xylenes, mercaptans, and organic sulfides and disulfides may be subject to cracking on molecular sieves during regeneration. Under the acidic conditions of a typical mole sieve, the mercaptans may form higher molecular weight species, and these species may then dehydrogenate to form coke. Thus the mercaptans may be decomposed into the corresponding olefins (R′) and hydrogen sulfide:RSH+heat→R′+H2SIn the case of methanethiol, R′ may represent a methylene radical. Two methylene radicals may combine to form ethylene, which may then form any number of polymers. These polymers may likewise dehydrogenate to form coke. Coke deposition on the mole sieve hinders gas flow (higher pressure drop), and inactivates the sorbent by physically blocking the micropores of the solid. The coke formed from such decomposition reactions builds up over repeated thermal regeneration cycles, thereby hindering fluid flow through the molecular sieve bed, eventually rendering it ineffective. When this occurs, the molecular sieves can no longer be regenerated and must be replaced at potentially significant expense, including possibly expenses incurred from unplanned downtime for the hydrocarbon purification facility.
In addition to deactivation by coking, molecular sieves can also undergo thermal deactivation. For this reason, it would be desirable to use a low regeneration temperature. If the regeneration temperature is too low, however, the quality of the product obtained from the regenerated molecular sieves may be inadequate. That is, if the regeneration temperature is too low to sufficiently desorb contaminants on the molecular sieves, the absorptive capacity of the regenerated molecular sieves will be low and the natural gas product or other fluid product obtained by treatment with such molecular sieves will have a contaminant level that is unacceptably high.
Accordingly, embodiments of the present invention provide an improved method of removing contaminants from hydrocarbon streams using molecular sieves and an improved method of regenerating the molecular sieves while reducing the adsorption of carbon-containing contaminants and resulting coking. Other embodiments incorporate such improved methods in a method for producing liquefied natural gas (LNG).